The present invention relates to an apparatus and method for mixing multi-part materials under vacuum and for dispensing the mix from the apparatus.
Multi-part reaction materials are commonly mixed under vacuum to eliminate entrapped air and gaseous reaction byproducts from the mix. The mixing vessel can be evacuated after mixing, but before use, or during mixing. Certain materials have relatively shorter set times and higher viscosity. It is more difficult to remove gases from these materials for several reasons. First, bubbles formed during mixing must migrate through the viscous mix to reach a surface exposed to vacuum. Second, the vacuum level that can be used during mixing is limited by the boiling points of the parts, as in bone cement; the differential pressure attainable at a given altitude; and the capabilities of the apparatus that creates the vacuum.
A known method for mixing bone cement to increase the rate of entrapped gas elimination comprises pre-evacuating the space surrounding one part of the cement prior to mixing, and then introducing the other part into the evacuated space. This method is less than completely satisfactory, however, because the turbulence caused by the second part filling the evacuated space can cause bubbles and voids to form in the mix.
It is also known to force a first part to replace the interstitial air in a second part. The air present in second material is displaced; but the method does not quickly and thoroughly mix the two materials, remove the gaseous reaction byproducts, or compensate for temperature.
It is important in the mixing of certain multi-part materials, that the materials be quickly combined and that the mix, de-air and de-gas steps be performed quickly, so as to protect the mix from outside contamination. Avoiding contamination is especially important in the mixing of bone cement under sterile surgical conditions. Generally, bone cements have a mix schedule, from the addition of a liquid monomer to a powder component, of one minute for mixing followed by one minute of pot life, before the mix needs to be injected. Accordingly, the efficiency of the mixing step is very important.
Some known methods of mixing bone cements comprise mixing materials within a container from which the mix is subsequently extruded, and applying a vacuum to the container during the mix, or both during and after the mix. Known methods agitate the bone cement mix to enhance mixing within the time constraints of the mixing process. Flat plate agitators having various hole patterns have been used to accomplish the mix. The mixed material can be extruded from the mixing chamber through the plate. Mixing only occurs when the agitator is moved in and out of the mixing chamber. Rotary motion repositions the holes for axial plunging, but provides only minimal mixing.
Another important consideration in regard to multi-part reaction materials is that the parts be readily accessible and easily handeable before and during the mixing procedure.
Bone cement liquid monomer is an example of a component of a multi-part reaction material that is difficult to handle. The monomer is usually delivered by the bone cement manufacturer in a glass vial, which is broken to add the monomer to the powder component. The monomer may not always be stored within the container in which the mixing is performed, and may be supplied separately from the device. This can make the mixing procedure more difficult to perform.
During the mixing of multi-part reaction materials under vacuum, gases can be evolved. These gases can be toxic to humans, and so it is important to prevent these gases from escaping into the environment.
Accordingly, there is a need for an apparatus for mixing and dispensing multi-part reaction materials that (i) can rapidly combine the materials to meet mix schedules, (ii) does not cause cavitation in the mix, reducing the entrapment of air and out-gassing; (iii) efficiently agitates the reaction materials throughout substantially the entire mixing chamber; (iv) can quickly establish and also maintain a vacuum in the mixing chamber; (v) exposes a large amount of surface area of the mix to vacuum during agitation, enabling trapped air and reaction gases to escape from even viscous mixes.